179 



plu noint na exhibited by numerous crystalline substances, namely 

 H-currence of the above mentioned optical anomalies and the 

 rotatory power in uniaxial crystals belonging to tin- trigon.-il, i 

 il, and hexagonal systems. 



( )ptical anomalies have been observed in a gradually increasing 

 number of crystals ever since Brewster *) in the beginning of the 

 niiu ti'i nth century first discovered and studied them. More especially 

 the symmetry of the optical behaviour of such crystals appeared 

 to be appreciably lower than that of their external forms; or, what 

 is another view of the same fact : their geometrical form is evidently 

 higher symmetrical than that of their internal molecular structure. 



Thus many crystals of the cubic system are birefringent, and, 

 in striking contrast to what might be expected, they act powerfully 

 upon transmitted polarised light; tetragonal and hexagonal crystals 

 are notoriously biaxial, and show optical phenomena analogous to 

 those to be expected in rhombic, monoclinic, or triclinic crystals; etc. 



The apparently cubic minerals: boracite, leucite, perowskite, fluor- 

 spar, diamond, garnet, analcite, etc., are in most cases distinctly bire- 

 fringent, and the same is true for many laboratory-products, such 

 as alums, the nitrates of barium, strontium, and lead, Schlippe's salt, 

 sodium-chlorate, sodium-bromate, etc., all crystallising in one of the 

 classes of the cubic system. The apparently tetragonal crystals of 

 potassium- ferrocyanide, of strychnine-sulphate etc., and of minerals 

 like idocrase, apophyllite, etc., are beyond all doubt optically biaxial. 

 The same is true for a great number of substances which, with respect 

 to their crystal-forms, must belong to the trigonal or hexagonal 

 systems, as for instance : quartz, turmaline, chabazite, sodium-periodate, 

 beryll, apatite, the dithionates of potassium, rubidium, caesium, 

 calcium, strontium, and lead, and many other chemical compounds. 



The disparity between the optical character of such crystals and 

 their geometrical appearance is therefore an indisputable fact, and 

 one even of frequent occurrence ; and a very great number of highly- 

 interesting investigations have been made with the purpose of eluci- 

 dating the causes of this striking discordance. The work done in this 

 field has chiefly led to two different standpoints in the explanation 

 of the phenomena considered. One class of investigators regards 



1) D. Brewster, Phil. Trans. London, /. 187. (1814); Trans. R. Soc. 

 Edinb. 8. /. 155. (1817); Phil. Trans. London,/. 199. (1818); Trans. R. Soc. 

 Eclinb. 9. /. 139. (1821); //. 317. (1823); 10. 187. (1826); Edinburg Phil. Journ. 

 1. 1. (1819); 8. 98. (1820); 5. 217. 218. (1821); Phil. Mag. 7. 245. (1835); etc. 



